The present invention relates to an optical deflector which reflects light from a light source and scans the reflected light.
Japanese Unexamined Patent Publication No. 64-2015 discloses an example of a conventional optical deflector the arrangement of which is illustrated in FIGS. 21A and 21B. FIG. 21A is a perspective view of the optical deflector and FIG. 21B is a cross-sectional view. In these figures, 1 denotes a movable plate having a reflecting means on its one surface, 2 denotes a hinge member of torsion bar type formed of a metal planar plate, 3 denotes a support member fixing both ends of the hinge member 2, 40 denotes a magnet, 6 denotes a first coil fixed to the hinge member 2 to face the magnetic poles of the magnet 40, 8 denotes a second coil fixed to the first coil, 9 denotes a yoke having a magnetic gap where the first coil 6 and the second coil 8 are positioned and forming a closed magnetic circuit, and 10 denotes a third coil fixed to the yoke 9 in the vicinity of the first coil 6.
In the optical deflector thus arranged, when an AC current is applied to the first coil 6, interaction between the current in the first coil 6 and a magnetic field produced by the magnet 40 exerts a Lorentz force on the first coil 6 in the direction of thickness of the movable plate 1. As a result, the movable plate 1 is rotated with its rotation center being the direction parallel to the movable plate plane in the hinge member 2.
The second coil 8 that rotates simultaneously with the movable plate 1 cuts the flux of a magnetic field produced from the magnet 40 and an induced electromotive force is produced across the second coil which is proportional to the rate at which the flux is cut. In addition to the induced electromotive force, an electromotive force is produced in the second coil 8 by mutual induction associated with the varying current in the first coil 6. This is the case with the third coil 10. To reduce the induced electromotive force produced in the second coil 8 by mutual induction, the difference in induced electromotive force due to the varying current in the first coil 6 between the second and third coils 8 and 10 is subjected to negative feedback to the first coil 6. The induced electromotive force produced in the second coil 8 which has the induced electromotive force associated with the varying current in the first coil 6 reduced in that manner serves as the velocity signal of the movable plate 1 and the second and third coils 8 and 10 function as means for controlling the light deflecting angle of the movable plate 1. With this optical deflector, laser light is directed to the movable plate 1, allowing reflected light to be scanned.
Japanese Unexamined Patent Publication No. 10-90625 describes another example of a conventional optical deflector.
FIG. 22 shows the planar configuration of this optical deflector, which comprises a movable plate 21, an elastic member 22, a support member 23, a driving coil 24, a sensing coil 25, driving coil electrode pads 26, sensing coil electrode pads 27, and a permanent magnet 28. Here, the movable plate 21, the elastic member 22, the support member 23, the driving coil 24 and the sensing coil 25 are integrally formed by means of semiconductor manufacturing techniques. The movable plate 21, which is formed mainly from a Si substrate of a high-stiffness material, is formed on its one side with the coils 24 and 25 and has a reflective planar surface as reflecting means on its opposite side.
The elastic member 22, which is formed mainly from an organic material such as polyimide, supports the movable plate 21 so that it can move in the direction of thickness. The support member 23, formed from the same substrate as the movable plate 21, fixes the elastic member 22. The driving coil 24 and the sensing coil 25 are made mainly from a metal such as Al, Cu, or the like and have two or more turns formed in as outside a portion of the movable plate 21 as possible. Both the driving coil and the sensing coil are placed almost vertically with respect to each other and an insulating layer is formed between the coils, thus providing isolation between the coils. The driving coil electrode pads 26 and the sensing coil electrode pads 27 are formed on the support member 23. The coils both pass through the elastic member 22 and terminate at the support member 23. The permanent magnet 28 is magnetized perpendicular to the direction of current flow in that portion of the driving coil 24 which is opposed to the magnet. To increase the driving force and to increase the output level of the detected signal, the magnet is placed in the vicinity of the movable plate 21, the driving coil 24, and the sensing coil 25.
In the optical deflector thus arranged, when an AC current is applied to the driving coil 24, interaction between the current in the driving coil 24 and a magnetic field produced by the magnet 28 exerts the Lorentz force on the driving coil 24 in the direction of thickness of the movable plate 21. The Lorentz force causes the movable plate 21 to set up both a translational motion in the direction of its thickness with the boundary between the elastic member 22 and the support member 23 as the fixed end and a rotating motion in the direction of its direction with an axis of the elastic member 22 in the thickness in which the magnet 28 is magnetized as the axis of rotation. The sensing coil 25 that translates and rotates simultaneously with the movable plate 21 cuts a magnetic flux produced by the permanent magnet 28 with the result that an induced electromotive force is produced across the sensing coil 25 which is proportional to the rate at which the sensing coil 25 cuts the magnetic flux. The induced electromotive force produced in the sensing coil 25 is used as a velocity signal of the movable plate 21 and the sensing coil 25 is used as means for controlling the angle of rotation of the rotatable pate 21. This optical deflector can scan the reflected light, which is obtained by irradiating light, such as laser light to the plate 21 that is translating and rotating.
However, since the signal of the sensing coil as detecting means used in the conventional techniques is a velocity signal, the position of the movable plate is not detected precisely. The integration of the velocity signal would produce a position signal, which however, might deviate from the precise position of the movable plate. In particular, when the frequency of driving current in the driving coil is low, the signal produced is very small in magnitude; thus, there is the strong possibility that the integrated signal may differ from the displacement signal of the movable plate. Further, with DC current applied, precise detection is difficult. Thus, when the movable plate is used as positioning means, such driving coil-based detect means as used in the conventional techniques cannot be used.
There is a method by which an elastic member is formed with a strain gauge made of a piezoresistive element and a signal is detected through bending and torsion of the elastic member. However, the deformation of the elastic member does not necessarily correspond to the displacement of the movable plate.